Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor manufacturing, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels.
FIG. 1 depicts a conventional ion implanter system 100. The ion implanter 100 includes a source power 101, an ion source 102, extraction electrodes 104, a 90° magnet analyzer 106, a first deceleration (D1) stage 108, a 70° magnet analyzer 110, and a second deceleration (D2) stage 112. The D1 and D2 deceleration stages (also known as “deceleration lenses”) are each comprised of multiple electrodes with a defined aperture to allow an ion beam to pass therethrough. By applying different combinations of voltage potentials to the multiple electrodes, the D1 and D2 deceleration lenses can manipulate ion energies and cause the ion beam to hit a target workpiece 114 at a desired energy. A number of measurement devices 116 (e.g., a dose control Faraday cup, a traveling Faraday cup, or a setup Faraday cup 122) may be used to monitor and control the ion beam conditions.
The ion source 102 is a critical component of the ion implanter system 100. The ion source 102 is required to generate a stable and reliable ion beam 10 for a variety of different ion species and extraction voltages.
FIG. 2 depicts a typical embodiment of an ion source 200 that may be used in the ion implanter system 100. The ion source 200 may be an indirectly heated cathode (IHC) ion source, which is typically used in high current ion implantation systems. The ion source 200 comprises an arc chamber 202 with conductive chamber walls 214. At one end of the arc chamber 202 there is an indirectly heated cathode (IHC) 203 having a cathode 206 with a tungsten filament 204 located therein. The tungsten filament 204 is coupled to a first power supply 208 capable of supplying a high current. The high current may heat the tungsten filament 204 to cause thermionic emission of electrons. A second power supply 210 may bias the cathode 206 at a much higher potential than the tungsten filament 204 to cause the emitted electrons to accelerate towards and heat the cathode 206. Heating the cathode 206 causes the cathode 206 to emit electrons into the arc chamber 202. A third power supply 212 may bias the chamber walls 214 with respect to the cathode 206 so that the electrons are accelerated at a high energy into the arc chamber. A source magnet 224a may create a magnetic field B inside the arc chamber 202 to confine electrons, and a repeller 216 at the other end of the arc chamber 202 may be biased at a same or similar potential as the cathode 206 to repel the electrons. A gas source 218 may supply a dopant species (e.g., BF3 AsH3, GeF4) into the arc chamber 202. The electrons may interact with the dopant species to produce a plasma 20. An extraction electrode (not shown) may then extract ions 22 from the plasma 20 through an extraction aperture 220 for use in the ion implanter 100.
A problem that currently exists in conventional ion implantation is that conventional ion implanters cannot effectively implant dopant species at both low energies (e.g., sub-kev) and high energies (e.g., keV). For example, conventional ion implanters utilizing an IHC ion source are relatively inefficient at transporting low-energy ion beams due to space charge within the ion beams. In general, the lower the energy, the greater the space charge problems.
One approach that has been proposed to solve the problem of low-energy boron implantation is molecular beam ion implantation. That is, instead of implanting an ion current I of atomic B+ ions at an energy E, a decaborane molecular ion (B10Hx+), for example, is implanted at an energy 10×E and at an ion current of 0.10×I. Although the resulting implantation depth and dopant concentration (dose) of both approaches have been shown to be generally equivalent, the decaborane implantation technique may have significant potential advantages. For example, since the transport energy of the decaborane ion is ten times that of a dose-equivalent boron ion and the decaborane current is one-tenth that of the boron current, space charge forces may be substantially reduced when compared to monatomic boron implantation.
While BF3 gas is used by conventional ion sources to generate B+ ions, decaborane (B10H14) must be used to generate the decaborane ion B10Hx+. Decaborane is a solid material which has a significant vapor pressure, on the order of 1 Torr at 20° C., melts at 100° C., and decomposes at 350° C. To be vaporized via sublimination, decaborane must be vaporized below 100° C. and must operate in an ion source whose local environment (e.g., chamber wall and chamber components) is below 350° C. to avoid decomposition. In addition, since the decaborane molecule is relatively large, the molecule easily disassociates into smaller components, such as elemental boron or diborane (B2H6), when subject to charge-exchange interactions within plasma. Therefore, it may be understood why conventional ion sources are not reliable in commercial production. Furthermore, ion source vaporizers typically cannot operate reliably at the low temperatures required for decaborane, due to radiative heating from the ion source to the vaporizer that causes thermal instability of the molecules. For example, vaporizer feed lines easily become clogged with boron deposits from decomposed vapor as the decaborane vapor interacts with their hot surfaces.
Additionally, a conventional ion source is generally incompatible with decaborane ion implantation and/or ion implantation using any other molecular species. Thus, a conventional ion source may only be satisfactory for atomic and small molecular ions.
While conventional ion sources, such as duoplasmatrons, may consist of two plasma regions: a cathode plasma (e.g., between a cathode and an intermediate electrode (IE)) and a high-density plasma (e.g., between the IE and an anode), the plasma is compressed by a double layer into an IE channel and by an axial magnetic field. Although a high axial magnetic field improves the yield of multiply charged ions, this two-region ion source cannot produce sufficiently high intensity electron beams because of high-frequency instabilities and leads to poor ionization of atomic species. As a result, a conventional ion source cannot reliably be used in high current ion implantation situations. Furthermore, in this example, filament lifetime is relatively short because of sputtering of heavy ions.
In addition, operating temperatures of an IHC ion source are typically 800° C. to 2300° C. from the chamber body to the heated cathode. Such extremely high temperatures shorten the performance and lifetime of IHC ion sources. As a result, the performance degradation and short lifetime of IHC ion sources greatly reduce the productivity of ion implanters. While these temperatures are generally required for thermal dissociation and atomic creation during implantation, they are not conducive for producing large molecular ions.
In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current ion source technologies.